In a conventional internal combustion engine for automotive vehicles, a fuel and air mixture is provided in correct proportions and a spark is used for igniting the air/fuel mixture. The spark is timed in relation to the position of the pistons in the engine cylinders to generate maximum torque while avoiding abnormal combustion of the air/fuel mixture. The variables that influence the optimum engine spark timing for any given operating condition include engine speed, manifold pressure, coolant temperature, intake air temperature, ambient pressure, and fuel octane. The correct spark timing based upon the instantaneous values for these variables is stored in a look-up table in the memory of a microprocessor, which forms a part of the electronic engine control system.
One of the chief parameters to be considered in such control systems is borderline spark knock. A spark knock in spark ignition engines produces a characteristic pinging sound that is audible to the human ear if the knock intensity is loud enough. Borderline knock is defined as the knock intensity level at which the pinging sound is barely discernible to the trained ear of a person riding within the vehicle.
The purpose of a control system using a knock detector is not to eliminate knock. To the contrary, engine performance suffers when knock is eliminated. Although, excessive knock can damage the engine. Thus, it is desirable to have knock present to some extent. On the other hand, audible knock is aesthetically objectionable. It is thus desirable to control engine operation so that knock is allowed, particularly at wide open throttle conditions, but is not permitted to become loud enough to be heard by the vehicle occupants. A typical control strategy distinguishes between acceptable and unacceptable levels of knock. The control system adjusts the spark advance until an acceptable level of knock is achieved. Accurate control of knock permits the engine to be calibrated closer to the optimum ignition timing. The borderline knock level represents the maximum allowable knock intensity.
Listening for borderline detonation is an integral part of the mapping and calibration of spark ignited engines. Under most operating conditions, if spark timing is advanced far enough the engine will begin to knock audibly. Borderline refers to the spark timing at which the knock intensity is just at the threshold of audibility.
Listening for the knocks is done both in operating vehicles and in dynamometer cells by trained human operators. In dynamometer cells which have separate control rooms, the engine sounds are picked up with a microphone and heard either through headphones or a loudspeaker by the human operator. The knocks must be detected in the presence of a variety of non-combustion related sounds from the engine (e.g., rod knock or piston slap) as well as other sounds present in the test cell or vehicle. These other sounds may either mask the knocks or, in the case of mechanical sounds such as jack shaft rattle, may actually mimic the knocks.
Prior art systems and methods for detecting and controlling borderline detonation is for human operators to listen carefully to the engine while adjusting the spark timing. The human operators subjectively estimate the borderline knock.
What is needed is to automate the prior art "human-in-the-loop" systems and methods for detecting and controlling borderline by real-time analysis of the knocks. Such an automated method would imitate human operators listening to engine sounds as a function of spark timing.